Screening device

ABSTRACT

A treatment chamber for purification and fractionation of materials suspensions, particularly fibrous suspensions under pressure, and wherein the treatment chamber includes a rotary screening drum provided with pulsation members, the drum also including screening holes located on the back-side of, or immediately behind, the pulsation members as viewed in the direction of drum rotation.

The present invention concerns a device for purification andfractionation of materials suspensions, notably fiber suspensions inpulp and/or paper mills and relates to screens of closed centripetaltype.

According to the invention the device includes a screening drum equippedwith a pulsation organ of a kind being part of the device as describedin the Swedish patent application 2528/71 or similar devices.

In the device the injected matter is, according to the above patentapplication, introduced under the required overpressure into anexterior, ring-shaped chamber from where the fiber suspension firstflows inwards through a rotating drum, provided with screen openings(perforations, slits etc.) as well as with pulsation organs arranged indifferent groups relative to the screen openings, whereupon remainingsuspension is further purified/screened inwards through a concentricinner screening drum, rotary or stationary. The accepted matter is takenfrom the interior of the inner screening drum. Rejected matter is takenpartly as coarse reject (containing knots and other coarser woodimpurities as well as sand and other heavy particles) from the outerring-shaped chamber, and partly as fine reject (containing minor, shortor long wood impurities as well as other minor heavy particles to theextent that they have not earlier been sorted out) from the spacebetween the two screening drums.

One of the purposes of this invention is by aid of the above procedureand equipment to achieve a purification and fractionation effect withregard to wood impurities as well as heavier particles as has not untilnow been possible to achieve with one single unit equipped withscreening facilities. The high purification effect with regard to heavyparticles, both large and small, in most cases enables the dispensingwith the power-consuming hydrocyclones in the screening positions forunbleached pulp of different kinds as well as for purification of and asblocking functions against impurities materializing during themanufacture of different pulps before pulp vats and paper-machines.

When using a high-speed rotary screening drum provided with bowl-shapedpulsation organs according to pat. appl. 2528/71, a local pressure waveensues at and close to the front of each pulsation organ as respectsdirection of rotation, as well as a suction wave to the leeward of theseorgans. The through-flow inwards of the suspension through the screenopenings nearest to the front of the pulsation organ concerned isfacilitated, the opposite being the case for the corresponding openingson the respective leeward sides. Since the volume capacity of theapparatus is large and, as a result, the flow-speed of the suspensioninwards through the screen openings considerable, it is impossible,despite the great centrifugal effect in this case, to prevent part ofthe heavier impurities to pass through the screen openings inwardsthrough the pulsation drum. After this passage, they are still subjectedto a high centrifugal effect and collect outwards on to the inside ofthe drum.

One fundamental characteristic of this invention is the devising andarrangement or grouping of the pulsation organs in a way to cause areturn action outwards through a minor number of the screen openings ofthe drum. Sand particles, rust flakes and other impurities of higherspecific weight or higher sedimentation rate than that of the pure fiberare here returned outwards. This return is of the greatest importancefor the attaining of optimum purification and fractionation effect usingone single apparatus. The fine reject from the ring-shaped space behindthe pulsation drum will in this way have a very low content also of lessheavy impurities like sand, rust and the like, the result being thatthis reject can be refined with less risk and wear to the milling anddefibrating organs and then be returned to the injecting matter to thedevice according to the invention. Said return takes place without anyharmful effects of accumulation of milled-down or defibrated woodimpurities in the subsequent fine-screening through the inner screeningdrum.

Below examples show different designs of pulsation organs in combinationwith screen openings to show the invention.

FIGS. 1-28 show different species of pulsation organs according to theinvention and FIGS. 29 and 30 show two alternative screen devicesaccording to the invention.

FIGS. 1 and 2 display the mantle of a pulsation drum, made out ofsheetmetal, where in FIG. 1 part of the mantle is extended in plane,FIG. 2 being a section along the line A -- A in FIG. 1. This is ahorizontal section of the drum which has a vertical center shaft. Thedrum is intended to rotate as shown by the arrow to the left in FIG. 2.The sheet-metal mantle 1 is provided with so-called eye-lidperforations, whose front sides 2 and 3 in the direction of rotationconstitute pulsation organs 4 on the outside of the drum and 5 on theinside of the drum.

FIG. 3 shows screen openings 6 of pulsation organ 4 viewed from sectionB -- B in FIG. 1. When the drum rotates quickly, a pressure wave ensuesin the suspension on the front sides of pulsation organs 4 and 5 and asuction wave on their leeward sides. The inwards flow of the suspensionthrough the drum is thus counteracted in respect of screen openings 6 bypulsation organs 4 and are facilitated in respect of screen openings 7by pulsation organs 5. The strength of the pressure and suction wavesdepends, besides by the design of the pulsation organs, mainly on theperipheral velocity of the drum relative to the peripheral velocity ofthe suspension in the liquid layers in question. At the relatively highperipheral speeds of the drum here concerned, range 7 - 15 m/sec., thedesign of the pulsation organs in respect of propelling effect on thesuspension is of major importance to the pulsation effect and to theflow performance of the suspension through the screen openings atpulsation organs 4 and 5 respectively. The total of the propellingeffect of pulsation organs 4 and 5 respectively should be as small aspossible. By aid of the above shown pulsation organs 4 and 5, bothindividually equally large and of identical design, a sufficientdifference is achieved in the flow of the suspension through the screenopenings of the respective pulsation organs to cause heavier particles,that may have passed inwards through screen openings 7 at pulsationorgans 5, to be returned by the centrifugal force outwards towards theweaker inflow through screen openings 6 at pulsation organs 4, which arefewer in numbers, e.g. as shown by the FIGS. In order that such a returntake place the incoming overpressure in the suspension must not be sohigh that the flow speeds through the screen openings become excessive.

Pulsation organs 4 are shown in vertical rows or groups but may ofcourse be arranged in horizontal or oblique rows or groups.

FIG. 4 shows a pulsation organ 4 obliquely arranged relative to theother pulsation organs. The upwards inclination has i.a. a down-feedeffect on the spiralling movement of the suspension downwards along thedrum. A downwards inclination has a back-keeping effect. The inclinationcan be varied in respect of pulsation organs 4 and 5 or for groups ofthese, respectively.

FIG. 5 shows part of the drum in cast condition, extended in plane, andFIG. 6 is a section thereof along the line C -- C in FIG. 5. The drum isdesignated 8 and intended to rotate to the left in FIGS. On the upperside (the exterior of the drum) is pulsation organ 9 with front side 10reversedly placed as viewed in the direction of rotation, andconstituting part of a cylinder. The back or leeward side 11 is planeand arranged in approximately 90° angle to the front side. As screenopening it has a hole 12 with the same inclination as the front side.Pulsation organs 13 on the opposite side may be similarly designed orhave a radial screen opening 14. Pulsation organs 9 and 13 are shown inequilateral, triangular partition with a free hole area of preferablyfully 20% of the mantle surface, 80% whereof preferably to fall on holes14. The pressure and suction waves, mentioned under the first designaccording to FIGS. 1 and 2, selfevidently are in certain relation to thesize of the pulsation organs. Behind each pulsation organ there ensues,at the peripheral speeds here concerned, a whirl with a powerful suctioneffect in the respective hole orifices on the leeward side of thepulsation organs. The size and height of the pulsation organs, theinclination of the front side and the leeward plane, and the number andgrouping of the pulsation organs are to be adapted according to thepulsation effects required for different types of fiber suspensions andtheir fiber concentration during the progressing purification sequencedownwards in the apparatus according to pat. appl. 2528/71, both in thechamber outside the drum and in the chamber inside it.

The difference in the inward flow speed of the suspension (downwards inFIGS.) through holes 14 and 12 respectively increases relative tocorresponding holes in FIGS. 1 and 2. The peripheral velocity of thedrum and the ingoing overpressure on the outside of the drum can be soadjusted that the flow speed in holes 14 approaches zero, alternativelyturns into a weak outward flow (upwards in FIGS.). The outward return,particularly of heavier impurities gets effective and the screenapparatus less sensitive to pressure and speed variations.

Hole and pulsator partition in the design according to FIG. 7 is thesame as in the design according to FIG. 5 - 6. Holes 14 in FIG. 6 havetheir correspondence in holes 15 which are cylindrical but inclined inthe rotation direction. Holes 16 are radial and tapering inwards. Holes12 in FIG. 6 have their counterparts in holes 17 which are conical. Theholes in FIG. 7 entail certain changes in the flows through therespective holes as compared with corresponding holes in FIG. 6 underotherwise identical conditions as regards ingoing overpressure andperipheral speeds for the drum which may be regarded as a kind ofcentrifugal pump functioning over the so-called confined point with theholes acting as a kind of blades.

In FIG. 8 the holes have been arranged at the side of the pulsationorgans and are radially directed. Holes 18 are cylindrical and holes 19conical.

FIG. 9 shows a pulsation organ 20 with two holes. The pulsation organsor a selected part thereof may be provided with more holes.

FIG. 10 shows a pulsation organ 21, obliquely arranged in the rotationdirection. The degree of inclination can be varied in respect of theouter and inner groups of pulsation organs and also within therespective groups; refer to above.

Below will be described a number of different designs of pulsationorgans which are particularly suitable for drums made of sheet-metal.

FIG. 11 shows a separate pulsation organ 22 welded to a perforatedsheet-metal drum 23, part of which is displayed in the plane. The screenopenings 24 are cylindrical as shown by the figure and/or conical insocalled equilateral, triangular partition. FIG. 12 shows a section ofthe pulsation organ along the line D -- D in FIG. 11. The pulsationorgan can also be riveted to the drum, possibly simultaneously with asimilar organ on the other side of the drum, or attached by other means.

FIG. 13 shows another pulsation organ 25, for instance made from asection of a pipe, also welded on to a hole-punched, preferablyperforated drum, part whereof is displayed in the plane.

FIG. 14 is a section of the pulsation organ along the line E -- E inFIG. 13. The screen openings are identical with those in FIGS. 11 and12. If the pulsation organ 25 is located in such way that its projectionagainst a plane normal to the rotation direction gets large, preferablylarger than its projection against the mantle surface of the screeningdrum, the disintegration effect of the device is facilitated which mayin certain cases be preferable.

Pulsation organ 26 in FIG. 15, shown as a section in FIG. 16 along theline F -- F, is bowl-shaped, as a part of a sphere, and pressed intononperforated areas of drum mantle 23, preferably prior to bending ofsame, the drum mantle being hole-punched as in FIGS. 11 and 13.

FIGS. 17 and 18 (the latter being a section along line G -- G in FIG.17) display a similar pulsation organ and drum, the pulsation organ herebeing provided with holes 27 on its leeward side and/or with reversedlylocated pulsation organs 28, similar to pulsation organ 25 in FIGS. 13and 14.

FIG. 19 shows a long pulsation organ 29, a cross-section whereof isshown in FIG. 20. The pulsation organ is pressed into non-punched partsof the mantle which is devised as shown in preceding FIGS.Alternatively, the pulsation organ can be provided with holes 30 on itsleeward side.

FIG. 21 displays pulsation organ 29 in smaller scale. The grouping isshown on a plane drum mantle.

FIG. 22 displays a similar pulsation organ 31, but here the perforationin the sheet-metal mantle is of so-called square type. Alternatively,the pulsation organs can be provided with holes 32 on the leeward side.

FIG. 23 shows the pulsation organ in smaller scale in a differentgrouping and with different lengths of pulsation organs 31 and 33, seenin the rotation direction as inclining upwards - backwards (31),respectively downwards - backwards (33).

FIG. 24 displays another grouping of pulsation organs 34 and 35, in thiscase hook-shaped with unequally long legs upwards and downwards thedrum.

FIG. 25 shows the same pulsation organ as in FIG. 24, but here thepulsation organs of the second and fourth rows in the figure are placedwith the open hook in the rotation direction. The pulsation organsaccording to FIGS. 15 - 25 can also be made in corresponding section tobe welded, riveted or otherwise attached to the drum mantle plane or asfinished drum, which is provided with different screen openings. In thisway an existing combination of pulsation organs can be supplemented incase of need.

FIGS. 26, 27 and 28 (FIG. 27 is a section along J -- J in FIG. 26) showa design which gives a favourable combination effect. The sphericalsectors 36 produce an efficient pulsation and the intermediate smallerand protruding parts 37 provide a desirable guiding effect on the axialmovement of the materials suspension as well as a certain pulsationeffect. If the mantle of the pulsation drum is moved in the direction ofthe arrow and the materials suspension is fed in from above, theaforementioned intermediate parts 37 will counteract the downwardmovement of the suspension if the angle alfa is larger than 0° andsmaller than 90°. By this the processing time is extended. If, on theother hand, the angle is chosen in the 90° - 180° range, theintermediate parts will contribute to an increase of the speed of thedownward feed. It is consequently possible by aid of each part toachieve both an efficient pulsation and a guiding of the movement of thesuspension across the screen surface.

In order that the above effect be achieved with the design according toFIGS. 26 - 28 is required that the pulsation/guiding organ is of theextension as to length and height as is required for achieving theintended effect. Since the height extension of the guiding organ islimited because of the risk of jamming caused by knots or majorparticles in the space outside the drum, the pulsation/guiding organmust be dimensioned in length with a certain minimum extension whichpreferably may be defined as a function of the width of the organ.According to the invention said relation is to be not less than 5:1,i.e. the length L measured along the mantle surface is to be not lessthan five times as large as the width B.

In FIG. 28 a modified design is shown. The guiding organ 38, which alsoproduces a certain pulsation effect, is arranged on both sides of theactual pulsation organ 39. In order to obtain a uniform design, allorgans are preferably to be devised either according to design 36 - 37or design 38 - 39, although both types may naturally be used on one andthe same drum, internally and/or externally. Screen openings 40 arearranged between and/or in organs 36 - 39 as shown in FIGS. 26 and 27.

By way of introduction was stated that the pulsation organs according tothe invention are particularly suitable for the knot-catching drumaccording to pat. appl. 2528/71. The pulsation organs can also be usedto advantage for a knot-catcher, preferably closed-type, i.e. ascreening apparatus intended exclusively for removing of knots andcoarse impurities. This design can, in principle, be obtained byexcluding the drum for finescreening in FIG. 2 of pat. appl. 2528/71.The screen device would in such case comprise only one screening drum(the outer drum with pulsation organs according to aforementioned pat.appl.) Under this alternative a special disintegration effect can beobtained if the pulsation drum is made more robust, preferably cast,e.g. as in FIGS. 6 - 10, and if likewise the mantle devised outside thedrum is made sturdier and provided with bulges which interact with thepulsation organs and the pulsation drum. At the same time an efficientprimary sorting is in this case obtained.

The above mentioned and shown designs and combinations of pulsationorgans and different groupings thereof only serve to illustrate theinvention and do not cover all designs, combinations and groupings thatare within the scope of the invention.

In FIGS. 29 and 30 are shown the two alternative screening devicesaccording to pat. appl. 2528/71 which are particularly suitable for thescreening drum, according to the invention, which is provided withpulsation organs.

The device according to FIG. 29 consists in its principal parts of ahighspeed rotary screening drum 1' and, concentrically with and outsideit, a highspeed rotary knot-catching and pulsation drum 2', andconcentrically with these drums a stationary mantle 3' attached with aholder 4' to a screen casing 5' which is fixedly attached to asupporting frame 6' which in turn is mounted on to a base stand 7'.

The knot-catching drum 2' is at the top attached to a cover 8' which isrigidly mounted on a rotary shaft 9'. The screening drum 1' is at thetop attached to a cover 10' equipped with low pump blades 11'. The cover10' is fixedly mounted on to a sleeve 12' mounted against the shaft 9'in radial bearings 13' and 14' and thrust bearings 15'. The sleeve 12'is externally carried in radial bearings 16' and 17' against bearingsupport 18' attached at the top to the supporting frame 6' and at itsbase inserted into the latter. Axially, the sleeve 12' is mounted to thebearing support 18' in thrust bearing 19' and a casing 20' fitted to thesupport 18' and provided with stuffing and sealing 20a'. The axial loadon the shaft 9' is transferred via a ring 21' to bearing 15' and isabsorbed by a casing 22' attached to the sleeve 12'. The radial bearingsuspension of the shaft 9' is reinforced by a radial bearing 23' whichis secured to shaft 9' by a screw 28' via the ring 21', wear linings 24'and 25', a driving pulley 26', and a washer 27'. Another driving pulley29' for the screening drum 1' is fixedly mounted on casing 22' using astuffing box sealing 30'.

The axial load on sleeve 12' is absorbed by a Seeger ring 31' andtransferred via the bearing 17', a ring 32', and bearing 19' to thecasing 20' screwed on to sleeve 12' which is inserted into a fitting inthe supporting frame 6'.

A nut 33' secures the cover 8', a wear lining 34' and the bearing 13' tothe shaft 9'.

The cover 8' is equipped with blades 35'.

The cover 10' is devised with stuffing boxes 36' and 37'.

Connecting on to the inner side of screening drum 1', pulsation blades38' are mounted on the supporting frame 6'. One or more of these blades38' may be hollow and devised for adding of flushing water outwardstoward screening drum 1' for extra cleaning of its screening organ atthe same time as the water being flushed through the drum dilutes thepulp on the outside of the drum to its full height.

The mantle 3' is in its upper part 39' coarsely perforated (φ 20 - 25mm) and in its lower part 40' perforated for adding of flushing orexcess water and provided with an outlet 41' for knots etc.

The screening drum is devised at the base with a seal 42', shown insocalled mechanical version.

The knot-catching and pulsation drum 2' is devised withpulsation-generating deformations 43' on its surface.

At the lower part of the drum 2' pressure water is introduced fordilution or flushing, partly through perforation 44' and, partly,through slots 46' at the two lower flanges of the drum 2'.

The apparatus is closed at its top with a cover 45'.

The driving pulleys 26' and 29' are driven via V-belt transmissions notshown by ditto separate electric motors, diamentally located in order toreduce the lower radial bearing loads.

Mode of operation:

The pulp, i.e. the incoming matter, is fed into the apparatustangentially at A, enters the ring-shaped waste chamber B from wherewaste etc. is drained off when necessary at G. The pulp is peripherallydistributed and flows inwards through the coarsely perforated mantlepart 39', is accelerated to suitable speed and required pressure by theblades 35', being simultaneously defibrated, enters in a dispersed statethe ring-shaped chamber D, flows against the centrifugal and pumpingforce, exerted by knot-catching drum 2', through the perforated zones ofthe latter where it is freed from knots, remaining pulp being dilutedwith pressure water added to chamber E at F through perforation 40' inthe mantle 3'. The knots are fed out through outlet 41' with suitablecounter-pressure at G.

The pulp, screened from knots, flows into the ring-shaped chamber N andinwards through the turbulent liquid layer proceeding through theperforation of the screening drum 1' which is locally cleaned by aid ofpressure blasts from blades 38'.

At the lower part of screening drum 1' pressure water for dilution ofremaining pulp is added at H to chamber J.

The remaining pulp, i.e. the reject, enters chamber K and is evacuatedat L at suitable counter-pressure by aid of which the volume/quantity isregulated.

The pulp passed through screening drum 1', i.e. the accepted pulp, isevacuated at M.

A simplified design is shown in FIG. 30 which is a central verticalsection of the apparatus, also in this case with a vertical shaft. Thecorresponding components etc. have the same figure and letterdesignations as in FIG. 29.

This design is principally intended for more easily screened andshortfibred pulps.

The principal difference between the two designs is that in the latterthe screening drum 1' is stationary.

The knot-catching and pulsation drum 2' is now attached to the cover 8'which is fixedly mounted on the shaft 9'. The cover 8' is provided withblades 35' and 11'.

On the upper flange of the screening drum a ring 10' is screwed on. Thescreening drum with ring is secured at the upper flange of the drum tothe flange on the supporting frame 6' and, at the same time, at thebase, on to the lower flange of the screen casing 5'. The ring 10' isscrewed on at the top to the upper flange of the supporting frame 6'.

The bearing sleeve 12' is at the base fitted into the supporting frame6' and screwed on to it at the top.

The shaft 9' is mounted on to the bearing sleeve 12', at the top inradial bearing 13' and at the base in radial bearing 14'. The axial loadon shaft 9' is absorbed by the ring 21', the thrust bearing 15' and acover 22' screwed on to the bearing sleeve 12' and equipped with a Stefaseal 30'.

The washer 27', the driving pulley 26', the wear lining 34' and thebearing 13' are secured against shaft 9' with the nut 33'.

The bearings are sealed at the top with stuffing boxes 36' and 36a'.

The mantle 3' is in its upper part coarsely perforated (φ 20 - 25 mm)and in its lower part 40' suitably perforated for the adding of pressurewater and provided with an outlet 41' for knots.

The knot-catching and pulsation drum 2' is devised withpulsation-generating deformations 43' on its surface. In the lower partof the drum pressure water is introduced through perforation 44' fordiluting purposes.

The apparatus is closed at the top with the cover 45'.

The driving pulley 26' is driven via a V-belt transmission, not shown,by a ditto electric motor.

Mode of operation:

The pulp, i.e. the incoming matter, is fed into the apparatustangentially at A, enters the ring-shaped waste chamber B from wherewaste etc. is drained off when necessary at G. The pulp is peripherallydistributed and flows inwards through the coarsely perforated mantlepart 39', is accelerated to suitable speed and required pressure by theblades 35', being simultaneously defibrated, enters in a dispersed statethe ring-shaped chamber D, is screened from knots by the perforatedzones of the drum 2' and is defibrated/deflocculated by the zones withprotrusions 43', and is diluted with pressure water through perforation40' in the mantle 3'. This perforation communicates with outlet 41'causing the knots to evacuate at suitable counter-pressure at G by meansof which the volume is regulated.

The pulp flows against the centrifugal and pumping force exerted by theknot-catching drum 2' and the liquid layer rotating on its inside,through the drum into the ring-shaped space N between the drums andpasses the boundary layer close to screening drum 1' and through thelatter.

At the lower part of screening drum 1' pressure water is added throughthe perforations 44' of screening drum 2' for diluting purposes. Theremaining pulp, the reject, enters chamber K and is evacuated at L atsuitable counter-pressure by aid of which the volume is regulated.

The pulp passed through screening drum 1' i.e. the accepted pulp, isevacuated at M.

In addition, different modifications of these devices are possible, e.g.the following:

a: in order to eliminate seal 42' in the design according to FIG. 29,the accepted pulp may be extracted at the top in which case the seal isplaced between the accepted pulp and the incoming matter, possibleleakage preferably being from accepted pulp to incoming matter. Thereare on this point different solutions, depending on whether theapparatus is operating in a pulp pipeline under comparatively highpressure or if the accepted pulp can flow off "freely" at the top.

b: in the design according to FIG. 30 with a stationary screening drum,the blades 38' may be rotary and driven at suitable speed in the sameway as the screening drum in the design according to FIG. 29.

c: in the design types referred to a vertical center shaft is used. Theymay however preferably use a horizontal shaft, especially if this wouldmake the design simpler and more rational.

d: when the apparatus is placed so far towards the beginning of theprocess that the fiber suspension is already screened from knots, thispurification stage is naturally deleted, which means that pressure waterneed not be added at F and that the knot outlet G does not have anyknot-removing function. The pressure water inlet and the knot outlet arein this case to be closed in some suitable way, e.g. by blank-flanging.It is of course also possible to make the construction cheaper bybuilding the apparatus without the abovementioned pressure-water inletand/or knot outlet. Should waste collection not be needed also wastechamber B can be deleted and possibly also the coarsely perforatedmantle part 39'.

I claim:
 1. A device for purifying and fractionating a materialsuspension, notably a fiber suspension, under pressure, said deviceincludinga treatment chamber having a circular cross-section, with aninlet for the suspension and outlets for reject and accept,respectively; a screening drum having a circular cross-section androtatable in the treatment chamber, said drum being provided withpulsation organs; said inlet being exterior of said drum, each pulsationorgan having a limited extension along the axial dimension of said drum,and a rounded front surface, said organ being adapted to createpulsations in the suspension layer close to the screening drum, saiddrum being perforate to provide an array of screen holes spaced on thesurface of said drum, the number of screen holes being considerablylarger than the number of pulsation organs and at least one screen holebeing located on the surface of each said pulsation organ at its leewardside, viewed in the direction of rotation of said drum, thereby causinga local return flow outwardly through said screening drum.
 2. A deviceaccording to claim 1, said pulsation organs being arranged on both theoutside and the inside of the screening drum.
 3. A device according toclaim 2, characterized by the screening drum being provided on itsinside with a greater number of pulsation organs than on its outside. 4.A device according claim 1, characterized by the pulsation organs beingat an angle to the rotation direction of the screening drum.
 5. A deviceaccording to claim 4, characterized by the pulsation organs beinginclined upwards, setting out from their front surface.
 6. A deviceaccording to claim 4, characterized by the pulsation organs beinginclined downwards, setting out from their front surface.
 7. A deviceaccording to claim 4, characterized by the pulsation organs beinginclined, in part upwards and in part downwards, setting out from theirfront surface.
 8. A device according to claim 1, characterized by saiddrum being of sheet metal and said pulsation organs being pressed out inthe sheet-metal of the screening drum.
 9. A device according to claim 1,characterized by the pulsation organs being prolonged and of a lengthfive times exceeding their width.
 10. A device according to claim 9,characterized by the pulsation organs being angular.
 11. A deviceaccording to claim 9, characterized by the pulsation organs beingprovided with at least one bowl-shaped protuberance which ascends abovethe rest of the pulsation organ.
 12. A device according to claim 11,characterized by a bowl-shaped protuberance being centrally located oneach pulsation organ.
 13. A device according to claim 11, characterizedby a bowl-shaped protuberance being located at each end of eachpulsation organ.
 14. A device according to claim 11, characterized bycertain pulsation organs having a centrally located bowl-shapedprotuberance and others having a bowl-shaped protuberance at each end.15. A device according to claim 1, characterized by the projection ofthe pulsation organs against a plane normal to the direction of rotationbeing larger than the projection of the organs against the surface ofthe screening drum.
 16. A device for purifying and fractionating amaterial suspension, notably a fiber suspension, under pressure, saiddevice includinga treatment chamber having a circular cross-section,with an inlet for the suspension and outlets for reject and accept,respectively; a screening drum having a circular cross-section androtatable in the treatment chamber, said drum being provided withpulsation organs; said inlet being exterior of said drum, each pulsationorgan having a limited extension along the axial dimension of said drumon its exterior surface, and a rounded front surface, said organ beingadapted to create pulsations in the suspension layer close to thescreening drum, said drum being perforate to provide an array of screenholes spaced on the surface of said drum, the number of screen holesbeing considerably larger than the number of pulsation organs and atleast one screen hole being located on the drum surface adjacent to saidpulsation organ on its leeward side viewed in the direction of rotationof said drum, thereby causing a local return flow outwardly through saidscreening drum.